In a scanning electron microscope (“SEM”), a primary beam of electrons is scanned upon a region of a sample that is to be investigated. The energy released in the impact of the electrons with the sample causes the emission of x-rays and secondary electrons, including backscattered electrons, from the sample. The quantity and energy of these x-rays and secondary electrons provide information on the nature, structure and composition of the sample. The term “sample” is traditionally used to indicate any work piece being processed or observed in a charged particle beam system and the term as used herein includes any work piece and is not limited to a sample that is being used as a representative of a larger population. The term “secondary electrons” as used herein includes backscattered primary electrons, as well as electrons originating from the sample. To detect secondary electrons, a SEM is often provided with one or more electron detectors.
The electron beam can also be used to initiate a chemical reaction at the sample surface. Process gases are used with charged particle beams to alter the work piece. “Beam chemistry” refers to chemical reactions initiated by a beam, such as a charged particle beam or a laser beam. “Electron beam chemistry” includes electron beam-induced deposition (EBID), electron beam-induced etching (EBIE) and electron beam-induced functionalization (EBIF) and is typically performed in a scanning electron microscope (SEM). In all of these electron beam processes, molecules of a precursor gas are adsorbed onto a work piece surface. An electron beam is directed at the work piece, and the electrons in the beam and the emitted secondary electrons dissociate the adsorbates, generating reaction products. In EBID, non-volatile reaction products remain on the substrate surface as a deposit, while volatile reaction products desorb. In EBIE, one or more of the precursor molecule decomposition products react with the work piece surface, generating volatile reaction products that desorb from the work piece, removing surface material. In EBIF, the electron beam-induced surface reaction changes the elemental or molecular species that terminates the work piece surface. Similar processes occur in ion beam-induced deposition (IBID), ion beam-induced etching (IBIE), and ion beam-induced functionalization (IBIF), although the much greater mass of the ions also causes material to be removed from the substrate by sputtering, that is, by momentum transfer from the energetic ions, without any chemical reaction. The mechanism by which the ion beam interacts with the adsorbate is thought to be different from the mechanism by which the electron beam reacts with the adsorbate.
In a conventional SEM, the sample is maintained in a high vacuum to prevent scattering of the primary electron beam by gas molecules and to permit collection of the secondary electrons. Beam chemistry is typically performed in the vacuum chamber using a gas injection system having a capillary needle that directs gas toward the impact point of the beam. The gas expands rapidly and while the local gas pressure at the surface is sufficient to support beam-induced reactions, the pressure in the rest of the sample chamber is sufficiently low that secondary electrons can be detected using a conventional detector, such as the scintillator-photomultiplier combination commonly referred to as an Everhart-Thornley detector.
Electron beam chemistry can also be performed with a work piece in an environment flooded with the precursor gas. One type of electron microscope in which the sample is maintained in a gaseous environment is an High Pressure Scanning Electron Microscope (HPSEM) or Environmental Scanning Electron Microscope. Such a system is described, for example, in U.S. Pat. No. 4,785,182 to Mancuso et al., entitled “Secondary Electron Detector for Use in a Gaseous Atmosphere.” An example is the Quanta 600 ESEM® high pressure SEM from FEI Company.
In an HPSEM, the sample is maintained in a gaseous atmosphere having a pressure typically between 0.01 Torr (0.013 mbar) and 50 Torr (65 mbar), and more typically between 1 Torr (1.3 mbar) and 10 Torr (13 mbar). The region of high gas pressure is limited to the sample region by one or more pressure-limiting apertures that maintain a high vacuum in the focusing column. By contrast, in a conventional SEM the sample is located in vacuum of substantially lower pressure, typically less than 10−5 Torr (1.3×10−5 mbar). In an HPSEM, secondary electrons are typically detected using a process known as “gas ionization cascade amplification” or “gas cascade amplification,” in which the secondary charged particles are accelerated by an electric field and collide with gas molecules in an imaging gas to create additional charged particles, which in turn collide with other gas molecules to produce still additional charged particles. This cascade continues until a greatly increased number of charged particles are detected as an electrical current at a detector electrode. In some embodiments, each secondary electron from the sample surface generates, for example, more than 20, more than 100, or more than 1,000 additional electrons, depending upon the gas pressure and the electrode configuration. In some embodiments positive gas ions or photons generated in the gas cascade are detected instead of electrons and used to generate an image. The term “gas cascade amplification imaging” as used herein refers to images generated using any combination of these three imaging signals. The term “gas cascade detector” as used herein refers to a detector that can be used to detect any combination of these three imaging signals.
As described in U.S. Pat. Pub. 2014/0363978 to Martin et al for “Electron Beam-Induced Etching,” which is assigned to the assignee of the present invention and which is hereby incorporated by reference, HPSEMs have several problems when used for beam chemistry. Impurities are introduced by desorption from surfaces inside the sample chamber, and by diffusion through o-rings typically used on SEM and HPSEM chambers. The impurities are comprised primarily of H2O, N2 and O2 and were not considered to interfere with conventional HPSEM imaging operations, which entails filling the chamber with a gas such as H2O vapor for the purpose of charge control and stabilization of vacuum-incompatible samples. The impurities do, however, interfere with beam chemistry because molecules such as H2O and O2 react with and cause the decomposition of deposition and etch precursors such as most organometallics, WF6, MoF6, Pt(PF3)4, XeF2, F2 and Cl2. The impurities also occupy surface sites at the sample surface thereby reducing the adsorption rate of precursor molecules used for beam chemistry; and cause the oxidation of materials such as W and Mo during deposition and thereby alter the composition and functional properties of the deposited material. The solution proposed by U.S. Pat. Pub. 2014/0363978 is to cool the work piece to a temperature near the boiling point of the precursor gas to provide high precursor surface coverage without condensation.
Another method used to provide improved control in a high pressure sample environment is the use of an “environmental cell” inside the sample chamber. By “environmental cell” is meant an enclosure for providing an environment around the sample, typically a different environment than that present in a sample chamber in which the environmental cell is located. An environmental cell can enhance control of the sample environment, reducing the concentration of gaseous impurities present during HPSEM processing, and reducing the volume and inner surface area of the HPSEM process chamber. PCT/US2008/053223, which is assigned to the assignees of the present application and which is hereby incorporated by reference, describes several configurations of environmental cells.
Another method to reduce contamination in a vacuum chamber is to use a cryotrap (also referred to as a “cold trap”), that is, a cold surface that condenses any gases in the vacuum chamber to improve the vacuum. Cryotraps are not used when a process gas is used, because the vacuum is intentionally degraded with the process gas. Cryotraps are also not used in HPSEM because the vacuum is intentionally degraded by the imaging gas, and the most common imaging gas is H2O vapor.
Two recent applications of beam chemistry are nanopatterning of graphene and diamond, which have unique electrical and optical properties. Gas-mediated EBIE is increasingly being used for rapid prototyping of functional structures in graphene and diamond because EBIE eliminates damage to the material produced by masking and ion irradiation. EBIE has been used to etch numerous carbon materials including graphene, carbon nanotubes, diamond, ultra nanocrystalline diamond (UNCD), and amorphous carbon-rich nanowires and films. At low electron beam energies (<˜30 keV), where atomic displacements by knock-on collisions between electrons and carbon are negligible, the removal of carbon is typically attributed to chemical etching (i.e., volatilization of carbon). The etching is generally ascribed to chemical pathways that involve O, H or OH radicals produced by electron induced dissociation of H2O, NH3 or H2 precursor molecules adsorbed to the surface of the etched material.
Applicants have found that a sample may be inadvertently etched even when no etch precursor gas is supplied. Thus, the sample can be degraded, for example, by unintentional etching during imaging by the electron beam in high vacuum, or when using a non-etching inert imaging gas, or during a deposition process. Such etching of graphene and diamond by low energy electrons have been attributed to mechanisms that include atomic displacements caused by knock-on, electron beam heating, sputtering by ionized gas molecules, and chemical etching driven by a number of gases that include N2. Y. Lan, et al., “Polymer-free Patterning of Graphene at Sub-10 nm Scale by Low-Energy Repetitive Electron Beam,” Small 10, 4778 (2014). D. Fox, et al., “Nitrogen assisted etching of graphene layers in a scanning electron microscope,” Appl. Phys. Lett. 98, 243117 (2011) and J. Niitsuma, et al., “Nanoprocessing of Diamond Using a Variable Pressure Scanning Electron Microscope,” Jpn. J. Appl. Phys. 45, L71 (2006). In particular, it has been reported that electron beam induced removal of carbon from graphene and diamond can be accelerated by introducing N2 into the vacuum chamber. These observations were very surprising and attributed to sputtering and chemical etching of carbon caused by nitrogen ions. When intentionally etching, the etch rate can vary under seemingly identical processing conditions. A method is needed to reduce inadvertent etching in order to produce consistent processing results for producing nanoscale structures, and to enable etch-free high resolution imaging of materials such as graphene, diamond and carbon nanotubes.